Numerous types of sound attenuating mufflers are known in the prior art. One type of muffler is a "straight through" muffler. A typical example of such a muffler is illustrated in U.S. Pat. No. 3,672,464 to Rowley et al. A convergent-divergent nozzle member is supported within a perforated outlet tube of the muffler and serves to attenuate sound generated by an internal combustion engine to which the muffler is attached.
Another type of prior art muffler is a combination muffler and air ejector unit. In such a muffler, two inlets to the muffler assembly are utilized. A first inlet communicates engine exhaust gases to the muffler and a second inlet communicates scavenged dirty air from the air cleaner during engine operation. Such a combined muffler and air ejector unit is illustrated in U.S. Pat. No. 3,419,892 to Wayne M. Wagner et al.
U.S. Pat. No. 4,111,279 to Sterrett discloses a muffler divided into a Helmholtz resonator chamber and a flow chamber. A first tube passes through the flow chamber and has an open end within the resonator chamber. Perforations or louvres through the first tube provide communication between the interior of the first tube and the flow chamber. A second imperforate tube extends through the resonator chamber and has an open end disposed within the flow chamber. Applicants have found that the use of a two-chamber system similar to the muffler system disclosed in the Sterrett patent, when constructed of a practical-size, does not exhibit sufficient sound attenuating properties. The need for mufflers with high noise attenuating capabilities has increased in recent years because of increasingly stringent governmental noise pollution regulations. For example, recent EPA regulation changes have lowered permissible sound levels on portable air compressors, which are commonly used in construction and road working applications.